This invention relates generally to radar systems, and more particularly to a calibrated moving target simulator of constant amplitude and precisely controllable Doppler.
The device of this invention is a calibrated test target for checking the performance of moving target indication (MTI) radars. It comprises a rotating dihedral reflector (quadrahedral) to simulate the return from a moving target with a constant radar cross-section (RCS). A number of methods exist in the prior art for simulating moving targets. One method is the injection of simulated target signals directly into the radar receiver. This method is commonly used to test MTI radars but does not include the effects of transmitter instabilities or scanning losses. In a second method, a microwave antenna is terminated by a waveguide short with a pin diode situated a distance in front of the waveguide short. The pin diode is modulated by alternately turning the pin diode on and off thus producing a discrete phase shift in the signal reflected back to the radar. The return signal provides a return at the simulated speed plus multiples of the simulated speed due to square wave modulation and, as such, cannot be used to determine signal loss at blind speeds. The return signal is also difficult to calibrate due to antenna voltage standing wave ratio and pin diode variables.
In a third method, a microwave signal is received in one antenna, modulated with a low frequency signal corresponding to the simulated target speed, amplified and retransmitted to a second antenna. The amplitude of the return is hard to calibrate due to the variability of all components in the system including antenna voltage standing wave ratios, amplifier gain and mixer conversion losses. The unwanted signal sideband is hard to eliminate due to the relatively low Doppler frequencies of the simulated target.
A fourth method involves the rotation of a trihedral or other reflector on an arm to induce motion of the reflector with respect to the radar. This method results in amplitude modulation and variation at the apparent target Doppler. An example is U.S. Pat. No. 4,370,654.
A fifth method involves modulation of one of the reflecting walls of a trihedral or other reflector by electrical or mechanical means. This method primarily produces amplitude modulation. Examples are U.S. Pat. Nos. 3,417,398; 3,308,464; and 2,917,739. Other prior art references include: A U.S. Army Electronics Command Technical Report entitled "An L-Band Moving Target Simulator" by John L. Kerr, November 1970. This report describes the development of an L-band moving target simulator which provides the Doppler frequencies required to simulate target radial speeds in the range of one-half miles per hour to thirty miles per hour. The design provides single sideband performance and generates the Doppler frequencies with reasonable rotational speeds.
However, none of the prior art methods contain the three most important attributes needed for a moving target simulator. These attributes are a precisely calibrated radar cross-section, a constant radar cross-section, and a precisely controllable Doppler frequency.